The manipulation of fluids to form fluid streams of desired configuration, discontinuous fluid streams, particles, dispersions, etc., for purposes of fluid delivery, product manufacture, analysis, and the like, is a relatively well-studied art. For example, highly monodisperse gas bubbles, less than 100 microns in diameter, have been produced using a technique referred to as capillary flow focusing. In this technique, gas is forced out of a capillary tube into a bath of liquid, the tube is positioned above a small orifice, and the contraction flow of the external liquid through this orifice focuses the gas into a thin jet which subsequently breaks into equal-sized bubbles via a capillary instability. In a related technique, a similar arrangement was used to produce liquid droplets in air
Microfluidics is an area of technology involving the control of fluid flow at a very small scale. Microfluidic devices typically include very small channels, within which fluid flows, which can be branched or otherwise arranged to allow fluids to be combined with each other, to divert fluids to different locations, to cause laminar flow between fluids, to dilute fluids, and the like. Significant effort has been directed toward “lab-on-a-chip” microfluidic technology, in which researchers seek to carry out known chemical or biological reactions on a very small scale on a “chip,” or microfluidic device. Additionally, new techniques, not necessarily known on the macro scale, are being developed using microfluidics. Examples of techniques being investigated or developed at the microfluidic scale include high-throughput screening, drug delivery, chemical kinetics measurements, combinatorial chemistry (where rapid testing of chemical reactions, chemical affinity, and micro structure formation are desired), as well as the study of fundamental questions in the fields of physics, chemistry, and engineering.
The field of dispersions is well-studied. A dispersion (or emulsion) is a mixture of two materials, typically fluids, defined by a mixture of at least two incompatible (immiscible) materials, one dispersed within the other. That is, one material is broken up into small, isolated regions, or droplets, surrounded by another phase (dispersant, or constant phase), within which the first phase is carried. Examples of dispersions can be found in many industries including the food and cosmetic industry. For example, lotions tend to be oils dispersed within a water-based dispersant. In dispersions, control of the size of droplets of dispersed phase can effect overall product properties, for example, the “feel” of a lotion.
Formation of dispersions typically is carried out in equipment including moving parts (e.g., a blender or device similarly designed to break up material), which can be prone to failure and, in many cases, is not suitable for control of very small dispersed phase droplets. Specifically, traditional industrial processes typically involve manufacturing equipment built to operate on size scales generally unsuitable for precise, small dispersion control. Membrane emulsification is one small scale technique using micron-sized pores to form emulsions. However, polydispersity of the dispersed phase can in some cases be limited by the pore sizes of the membrane.
While many techniques involving control of multi-phase systems exists, there is a need for improvement in control of size of dispersed phase, size range (polydispersity), and other factors.
An article entitled “Generation of Steady Liquid Microthreads and Micron-Sized Monodisperse Sprays and Gas Streams,” Phys. Rev. Lett., 80:2, Jan. 12, 1998, 285-288 (Ganan-Calvo) describes formation of a microscopic liquid thread by a laminar accelerating gas stream, giving rise to a fine spray.
U.S. Pat. No. 6,120,666, issued Sep. 19, 2000, describes a micofabricated device having a fluid focusing chamber for spatially confining first and second sample fluid streams for analyzing microscopic particles in a fluid medium, for example in biological fluid analysis.
U.S. Pat. No. 6,116,516, issued Sep. 12, 2000, describes formation of a capillary microjet, and formation of a monodisperse aerosol via disassociation of the microjet.
U.S. Pat. No. 6,187,214, issued Feb. 13, 2001, describes atomized particles in a size range of from about 1 to about 5 microns, produced by the interaction of two immiscible fluids.
U.S. Pat. No. 6,248,378, issued Jun. 19, 2001, describes production of particles for introduction into food using a microjet and a monodisperse aerosol formed when the microjet dissociates.
An articled entitled “Dynamic Pattern Formation in a Vesicle-Generating Microfluidic Device,” Phys. Rev. Lett., 86:18, Apr. 30, 2001 (Thorsen, et al.) describes formation of a discontinuous water phase in a continuous oil phase via microfluidic cross-flow, specifically, by introducing water, at a “T” junction between two microfluidic channels, into flowing oil.
Microfluidic systems have been described in a variety of contexts, typically in the context of miniaturized laboratory (e.g., clinical) analysis. Other uses have been described as well. For example, International Patent Publication No. WO 01/89789, published Nov. 29, 2001 by Anderson, et al., describes multi-level microfluidic systems that can be used to provide patterns of materials, such as biological materials and cells, on surfaces. Other publications describe microfluidic systems including valves, switches, and other components.
While the production of discontinuous fluids, aerosols, and the like are known, very little is known about discontinuous fluid production in microfluidic systems, i.e. the production of liquid-liquid and gas-liquid dispersions and emulsions. This may be due to the fact that precise control of fluid flow in microfluidic systems can be challenging.